January 3, 2013
by Marcus Woo

Caltech astronomers have estimated that the Milky Way Galaxy contains at least 100 billion planets.

(Phys.org)—Look up at the night sky and you'll see stars, sure. But you're also seeing planets—billions and billions of them. At least.

That's the conclusion of a new study by astronomers at the California Institute of Technology (Caltech) that provides yet more evidence that planetary systems are the cosmic norm. The team made their estimate while analyzing planets orbiting a star called Kepler-32—planets that are representative, they say, of the vast majority in the galaxy and thus serve as a perfect case study for understanding how most planets form.

"There's at least 100 billion planets in the galaxy—just our galaxy," says John Johnson, assistant professor of planetary astronomy at Caltech and coauthor of the study, which was recently accepted for publication in the Astrophysical Journal. "That's mind-boggling."

"It's a staggering number, if you think about it," adds Jonathan Swift, a postdoc at Caltech and lead author of the paper. "Basically there's one of these planets per star."

The planetary system in question, which was detected by the Kepler space telescope, contains five planets. The existence of two of those planets have already been confirmed by other astronomers. The Caltech team confirmed the remaining three, then analyzed the five-planet system and compared it to other systems found by the Kepler mission.

The planets orbit a star that is an M dwarf—a type that accounts for about three-quarters of all stars in the Milky Way. The five planets, which are similar in size to Earth and orbit close to their star, are also typical of the class of planets that the telescope has discovered orbiting other M dwarfs, Swift says. Therefore, the majority of planets in the galaxy probably have characteristics comparable to those of the five planets.

While this particular system may not be unique, what does set it apart is its coincidental orientation: the orbits of the planets lie in a plane that's positioned such that Kepler views the system edge-on. Due to this rare orientation, each planet blocks Kepler -32's starlight as it passes between the star and the Kepler telescope.

By analyzing changes in the star's brightness, the astronomers were able to determine the planets' characteristics, such as their sizes and orbital periods. This orientation therefore provides an opportunity to study the system in great detail—and because the planets represent the vast majority of planets that are thought to populate the galaxy, the team says, the system also can help astronomers better understand planet formation in general.

"I usually try not to call things 'Rosetta stones,' but this is as close to a Rosetta stone as anything I've seen," Johnson says. "It's like unlocking a language that we're trying to understand—the language of planet formation."

One of the fundamental questions regarding the origin of planets is how many of them there are. Like the Caltech group, other teams of astronomers have estimated that there is roughly one planet per star, but this is the first time researchers have made such an estimate by studying M-dwarf systems, the most numerous population of planets known.

To do that calculation, the Caltech team determined the probability that an M-dwarf system would provide Kepler-32's edge-on orientation. Combining that probability with the number of planetary systems Kepler is able to detect, the astronomers calculated that there is, on average, one planet for every one of the approximately 100 billion stars in the galaxy. But their analysis only considers planets that are in close orbits around M dwarfs—not the outer planets of an M-dwarf system, or those orbiting other kinds of stars. As a result, they say, their estimate is conservative. In fact, says Swift, a more accurate estimate that includes data from other analyses could lead to an average of two planets per star.

M-dwarf systems like Kepler-32's are quite different from our own solar system. For one, M dwarfs are cooler and much smaller than the sun. Kepler-32, for example, has half the mass of the sun and half its radius. The radii of its five planets range from 0.8 to 2.7 times that of Earth, and those planets orbit extremely close to their star. The whole system fits within just over a tenth of an astronomical unit (the average distance between Earth and the sun)—a distance that is about a third of the radius of Mercury's orbit around the sun. The fact that M-dwarf systems vastly outnumber other kinds of systems carries a profound implication, according to Johnson, which is that our solar system is extremely rare. "It's just a weirdo," he says.

The fact that the planets in M-dwarf systems are so close to their stars doesn't necessarily mean that they're fiery, hellish worlds unsuitable for life, the astronomers say. Indeed, because M dwarfs are small and cool, their temperate zone—also known as the "habitable zone," the region where liquid water might exist—is also further inward. Even though only the outermost of Kepler-32's five planets lies in its temperate zone, many other M dwarf systems have more planets that sit right in their temperate zones.

As for how the Kepler-32 system formed, no one knows yet. But the team says its analysis places constraints on possible mechanisms. For example, the results suggest that the planets all formed farther away from the star than they are now, and migrated inward over time.

Like all planets, the ones around Kepler-32 formed from a proto-planetary disk—a disk of dust and gas that clumped up into planets around the star. The astronomers estimated that the mass of the disk within the region of the five planets was about as much as that of three Jupiters. But other studies of proto-planetary disks have shown that three Jupiter masses can't be squeezed into such a tiny area so close to a star, suggesting to the Caltech team that the planets around Kepler-32 initially formed farther out.

Another line of evidence relates to the fact that M dwarfs shine brighter and hotter when they are young, when planets would be forming. Kepler-32 would have been too hot for dust—a key planet-building ingredient—to even exist in such close proximity to the star. Previously, other astronomers had determined that the third and fourth planets from the star are not very dense, meaning that they are likely made of volatile compounds such as carbon dioxide, methane, or other ices and gases, the Caltech team says. However, those volatile compounds could not have existed in the hotter zones close to the star.

Finally, the Caltech astronomers discovered that three of the planets have orbits that are related to one another in a very specific way. One planet's orbital period lasts twice as long as another's, and the third planet's lasts three times as long as the latter's. Planets don't fall into this kind of arrangement immediately upon forming, Johnson says. Instead, the planets must have started their orbits farther away from the star before moving inward over time and settling into their current configuration.

"You look in detail at the architecture of this very special planetary system, and you're forced into saying these planets formed farther out and moved in," Johnson explains.

The implications of a galaxy chock full of planets are far-reaching, the researchers say. "It's really fundamental from an origins standpoint," says Swift, who notes that because M dwarfs shine mainly in infrared light, the stars are invisible to the naked eye. "Kepler has enabled us to look up at the sky and know that there are more planets out there than stars we can see."

Related Stories

News flash: The Milky Way galaxy just got a little weirder. Back in 2011 astronomers were amazed when NASA's Kepler spacecraft discovered a planet orbiting a double star system. Such a world, they realized, would have double ...

(PhysOrg.com) -- NASA's Kepler mission has discovered the first Earth-size planets orbiting a sun-like star outside our solar system. The planets, called Kepler-20e and Kepler-20f, are too close to their star to be in the ...

In the search for Earth-like planets, it is helpful to look for clues and patterns that can help scientist narrow down the types of systems where potentially habitable planets are likely to be discovered. New research from ...

Hot on the heels of confirming one Kepler planet, the Hobby-Eberly Telescope announces the confirmation of another planet. Another observatory, the Nordic Optical Telescope, confirms its first Kepler planet as well, this ...

(Phys.org) -- The discovery of planets around other stars has led to the realization that alien solar systems often have bizarre features - at least they seem bizarre to us because they were so unexpected. For example, many ...

(Phys.org)—Two newly submitted studies verify 41 new transiting planets in 20 star systems. These results may increase the number of Kepler's confirmed planets by more than 50 percent: to 116 planets hosted in 67 systems, ...

The constellation of Virgo (The Virgin) is especially rich in galaxies, due in part to the presence of a massive and gravitationally-bound collection of over 1300 galaxies called the Virgo Cluster. One particular member of ...

(Phys.org)—A small team of researchers from the U.S. and Italy has found evidence of a naturally formed quasicrystal in a sample obtained from the Khatyrka meteorite. In their paper published in the journal Scientific Reports, ...

As a cosmic dust magnet, Saturn's C ring gives away its youth. Once thought formed in an older, primordial era, the ring may be but a mere babe – less than 100 million years old, according to Cornell-led astronomers in ...

so if 100 billion is "a staggering number, if you think about it", what is 16 Trillion...like the US national debt...160 galaxies worth of planets...truly staggering.

And this is only based on the cash method of accounting, which government reserves for itself. All large private corporations must by law use the accrual method, where you must show all significant future liabilities you have committed to as well. If the federal government had to show its unfunded future liabilities, the debt is actually closer to ONE HUNDRED TRILLION, which is a very conservative estimate. The officers of any publicly traded corporation that tried to avoid showing its future unfunded liabilities like this would be serving hard time, but we keep re-electing progressives who hide the true liabilities with the active participation of the "unbiased", "objective" "news" media. These future liabilities are caused by buying votes with promises of future chickens in every future pot.

Actually, Bush made trillions of dollars in tax cuts for the obscenely wealthy, and then fought a war in all the wrong ways, spending several trillion dollars "rebuilding" in nations that hate our guts to this day.

If not for the Bush tax cuts, the debt would only be about 12 trillion right now. Further, if not for the anti-strategic Bush strategy in Iraq and Afghanistan, the debt would probably only be about 10 trillion.

I would have confiscated teh oil fields in Iraq and their profits until the full cost of the war was paid, plus a 10% reprisal, and a 10 million dollar penalty for every man, woman, and child killed in the 911 attacks (plus all coalition soldiers killed during the war).

That's what the "Good guys" are supposed to do when terrorists and rogue nations attack.

1, kick their ass2, punish them after the war by forcing them to pay for the original damages plus expenses of the war.

And so, if we had implemented anything resembling the notion of Reprisal, we would have broken even on the war, and then gained the 10% penalty plus the 10 million dollars per casualty, and our economy would be back in business and booming, and the interest on our debts would only be 60% of what it is now, and we'd even have a budget surplus, most likely.

Our pussified western civilization "democracy lie" mentality is what is killing our nation.

By pussified I mean we give to our enemies when we should take, and we train our enemies when we should let them rot.

The "democracy lie" is the lie which says that democracy is somehow inherently good, and in this case the lie that said democratizing the middle east would solve the problems. It hasn't. Terrorism is actually back on the rise in Iraq!

At least for now we are doing the correct thing RE Syria, by staying out of there. Since neither side in the civil war is our friends, our strategy should be to let them deplete one another.

Funny, I thought Iraq had nothing to do with the terrorist attacks on the US. Confiscating their oil fields and making them pay for what was an illegal war mounted on the flimsiest of excuses (Bush and Blair were the only Weapons of Mass Deception found) doesn't sound right to me. Of course it is what has happened anyway with US and British companies awarded oil and gas contracts and you can bet those "good guys" will end up making a whole lot more money for themselves than has been spent on the war - otherwise what would the point of it all been?

Anyway aren't we all getting off topic a bit (or a lot)? At least 100 billion planets. That's very cool.

A "Rosetta stone"! And nice to see several independent methods converge on the same result!

Added to these 1-2 orbiting planets/star, there are microlensing results indicating 1 nomad planet/star, in concordance with planetary system formation modeling which routinely ejects some.

"The fact that M-dwarf systems vastly outnumber other kinds of systems carries a profound implication, according to Johnson, which is that our solar system is extremely rare. "It's just a weirdo," he says."

These systems are more like anything with some typical traits than those seen before, but still the observation is that all systems are unique. The planets and their final orbits mixes a lot as they migrate in, or out as in other cases (giants in our system, similar wide orbiting planets in other systems).

This is because the Monte Carlo simulations of inflation universes can be fitted to the CMB spectra and other data sources in combination, and the differences in baryonic and dark matter are explicitly handled.

The current debt is bad but it's not as bad as people make it out to be. As long as the economy and GDP contiue to grow we'll be fine. It's when the economy stalls that we begin to have problems. And I would also blame the folks that created most of this debt in the first place. A little hint, it's not the current president or congress either.

" If we continue to grow we will be just fine." What a relief! For a moment there I thought that the exponential function had consequences.I hereby formally protest and assert that I am affronted by the limitations of the speed of light.All that real estate going to waste.It is a pity that the native microbes would find us succulent. And if we so much as broke wind it would be party time. It would be on for young and old(microbes). Both ecosystems would be lowered to the lowest common denominator.That is if Panspermia is false.

That 67% does not include money the U.S. government owes to itself, which comes in at around 5 trillion, and puts the total at over 100% of GDP.

America's failure will continue for several reasons.

1. It's economy has since Reagan, become habituated on deficit spending, and can only be weened off of it slowly.

2. Unlike the world after WW2 when the U.S. was the only surviving economic power, the U.S. must now compete globally with nations that have strong competitive advantages in wages and reduced standards of quality, safety and environmental regulation.

3. Current deficits can not be funded with budget cuts without causing 40 million people to become unemployed overnight, and pushing the U.S. into a Grand Economic Depression that would last decades.

Reducing the deficit slowly through spending cuts and economic growth will see U.S. debt load double from what it is today. Debt service will climb to close to a trillion dollars a year.

4. Republicans refuse to act rationally on the debt, guaranteeing gridlock in trying to reduce it.

5. The plans Republicans have for reducing the debt if implemented guarantee that the U.S. will enter a Grand Economic Depression that will last decades. Reducing debt levels will be vastly more difficult in such an environment.

6. Interest rates will not stay low forever. When they start to rise, the U.S. bond rate will have to rise with international rates in order for the U.S. debt to be rolled over.

If interest rates double, then the U.S. $560 billion/Year will grow to double that, which is 1/3rd of all government spending.

Could the physorg posters keep the subject for at least five posts bellow each article? I'm afraid, it would exceed their mental if not intellectual capacity.

Agreed. These posts, and other similar ones on PhysOrg, just confirm to readers in the rest of the world that Americans have an overwhelmingly parochial mentality and that, whilst hurling playground insults at each other, they continually miss the bigger picture!

Could the physorg posters keep the subject for at least five posts bellow each article? I'm afraid, it would exceed their mental if not intellectual capacity.

Agreed. These posts, and other similar ones on PhysOrg, just confirm to readers in the rest of the world that Americans have an overwhelmingly parochial mentality and that, whilst hurling playground insults at each other, they continually miss the bigger picture!

This is because the Monte Carlo simulations of inflation universes can be fitted to the CMB spectra and other data sources in combination, and the differences in baryonic and dark matter are explicitly handled.

Wow... I stand "corrected"...? However, I believe the operative phrase in your quote was "with the current data"...

From what I can glean from the information returned by the Kepler Survey and others thus far is the thing that makes our solar system appear to be almost unique, and that is the presence of this Earth at a far enough distance away from its star to make life a more probable event. This suggests that the "worlds in collision" theory is probably true, because this would have re-distributed the planets into a new orbital configuration. If the asteroid belt were to have remained a single massive object, then Earth might now be too close to the sun to foster life. That says that something cosmically significant has to occur for life to begin on a planet around a star.

Of note is that this is a lower limit. Kepler has now ~ 18 000 candidates at ~ 98 % verified detection ability. [Arxiv paper.]

Given the ~ 11 000 stars with planets out of ~ 100 000, and the ~ 1 % coincidence of correct angle, one can estimate that about every star has planets.

An estimate out of Sun-like stars of ~ 400 stars (~ 0.4 % ability to see Venus from afar) with ~ 700 planets ( ~ 0.3 % ability to see Earth), and 8 planets out of 200 - 400 billion stars give a prior of ~ 2-3 trillion planets in the MW.

One can now fill in the outer planets by bumping up the lower limit by 2 orders of magnitude (110/4 stars, 5/2 planets) to a posterior of ~ 10 trillion planets! This is because systems are so closed packed compared to ours.

Our system is likely uncommonly low in planets.

With current WMAP data the universe is at least 10 times larger than the observable universe (and probably infinite). The OU have ~ 200 billion galaxies. Therefore at least ~ 10^27 planets exist.

so if 100 billion is "a staggering number, if you think about it", what is 16 Trillion...like the US national debt...160 galaxies worth of planets...truly staggering.

Naw, they've been playing with you. Just add a milliard to the dollar and it will be like the reichsmark of the 20s. 1 Papermark exchanged for 10^12 Reichsmarks (that is a trillion).

And as we all know, German hyperinflation was concocted to get Germans pissed off enough to accept national socialism, start the second phase of a worldwide war, and blame it on the Jews. And end up sacrificing 11M of themselves in the Process.

I wonder how many other planets this sort of thing is happening on? I wonder if there is another phony country named czechoslovakia out there somewhere?

We should also add in the observations of nomads of ~ 1/star. Another ~ 200 - 400 billion planets.

And nomads and their ice moons can be habitable for billions of years akin to our habitable ice moons (Europa, Enceladus et cetera).

@Whydening Gyre:

Current data is the resolution of WMAP for the CMB, a constant phenomena, and the resolution of auxiliaries like acoustic baryon oscillations (how clusters clump), also constant phenomena. WMAP has improved resolution, but the parameters have been stable. Next generation observations (Planck et cetera) can only improve resolution, and that is what they note.

Agreed. These posts, and other similar ones on PhysOrg, just confirm to readers in the rest of the world that Americans have an overwhelmingly parochial mentality and that, whilst hurling playground insults at each other, they continually miss the bigger picture!

Perhaps but on the upside they have discarded such effeminate and affectual words as 'whilst', as well as many useless vowels. This is progress, the kind that the US is to be commended for.

The actual eta could, with the posterior estimate of planets, be over 100 %.

All systems look individual, and even if our type of system is rare (few planets, only one habitable), it would have no implications on its formation.

The current system is nicely constrained by the Nice model and the molecular cloud formation model combined. No fragmenting collisions happened (cf Earth-Moon) but the asteroid belt is remains of the protoplanetary disk that didn't form planets or got scattered during the Jupiter-Saturn migration. Similar debris disks are observed in other systems and they too show that they are disk remains.

Tor.Your comment on this beiong the lower limit was so long that i couldn't do a "quote" on the whole thing! So just a quick response - ahhhh.... the widening gyre...Our level (definition) of what is observable is increasing at an ever increasing rate as well, it seems...

@TheGhostofOtto1923 aka FrankHerbertOtto. . . . .why are U telling lies about me and why are U saying that all these other people are me? U KNOW that there is only ONE RITCHIEGUY and that is ME. U have been looking for me in all those other people just so that U can pretend that U aren't a homosexual, but we both know that U have turned me on to the joys of suck suck and anal sex when U showed me your bigjuicycock at the motel where we stayed each nite.And now U are avoiding talking to me and U won't even call me. Why?U told me U love me and that we could get married when same sex marriage becomes legal.But now U are pretending that U don't know me even tho U called everyone RitchieGuy and they aren't me.Otto, please call me and let me back into your life, I don't want anyone else but U.

From what I can glean from the information returned by the Kepler Survey and others thus far is the thing that makes our solar system appear to be almost unique, and that is the presence of this Earth at a far enough distance away from its star to make life a more probable event. This suggests that the "worlds in collision" theory is probably true, because this would have re-distributed the planets into a new orbital configuration. If the asteroid belt were to have remained a single massive object, then Earth might now be too close to the sun to foster life. That says that something cosmically significant has to occur for life to begin on a planet around a star.

Life around such red dwarfs arent that rosy. Planets tidally locked to stars, extreme temps on each side and not enough light on visible spectrum for photosynthesis.. may be our solar system IS unique afterall

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.